Published German Application No. 28 29 210 describes a process wherein, in dust-collecting electrostatic precipitators, the dust which has been deposited on the collecting electrode walls is periodically removed to maintain the full collecting capacity. For that purpose, the collecting electrode walls are vibrated in known manner, e.g. by rapping means, so that the adherent dust layers are detached and drop into the underlying dust-collecting bins. During that cleaning, part of the previously deposited dust can be reagitated by the gas stream and can be carried by the gas stream out of the dust-collecting electrostatic precipitator.
In order to minimize the so-called rapping losses, a very low velocity is usually selected for the gas stream and a plurality of fields are arranged one behind the other although this involves high capital cost.
It is known to avoid the rapping losses in that shut-off flap valves or the like are provided at the entrance or exit ends of the gas passages and in case of need can be swung from a position of rest in which they are parallel to the gas stream, to an operative position in which they are transverse to the gas stream (see U.S. Pat. No. 2,554,247). As a result, one gas passage or a plurality of gas passages can be shut off for the duration of the mechanical cleaning (agitation of the collectors) so that there will be no gas flow and no dust can be reagitated.
But such mechanical shut-off means are costly and the considerable expense in many cases is not justified by the improvement of the separating capacity which can be achieved. The main disadvantage of such shut-off means is that the bearings of the movable parts are exposed to the hot gas stream and to the dust entrained thereby so that trouble often arises during operation and high maintenance and repair costs are involved in addition to the capital cost.
In the process known from Published German Application 28 29 210, these disadvantages are overcome in that an auxiliary gas flowing opposite to the normal direction of gas flow is injected adjacent to the collecting electrode wall to be cleaned during the cleaning period. This measure has been adopted because a gas flowing at a given rate and at a given velocity can be braked by a gas flowing at a much lower rate and at a higher velocity in the opposite direction and the rate of the opposing flow which is required can be calculated by means of the momentum theorem even if details of the turbulent mixing are not known. Model calculations have shown that a gas stream flowing at a velocity of, e.g., 1.5 m/s can be braked by an opposing stream under the pressure of 20 millibars and at a volume flow rate which is 1% of the volume flow rate of the stream to be braked.
However, even this known process still requires improvement. In modern dust collectors, at least two fields are usually arranged one behind the other in the direction of gas flow. Because dust is collected at highly different rates in the different fields--in a dust collector having three fields and a total collecting capacity of 99.9% of the dust content of the raw gas, about 90% are collected in the first field, 9% in the second and 0.9% in the third--the conditions for the periodic cleaning are usually separately adjusted for each field because the collecting electrode walls must be cleaned more often in the first field than in the last field although the differences are not as large as the differences between the dust collection rates because a classification is effected in multi-field dust collectors.
Under adverse conditions, such as a low dewpoint temperature, a high dust resistance or a high gas temperature, that known mode of operation is not satisfactory because the reagitation of the previously deposited dust will inevitably raise the dust content of the pure gas above a permissible limit and said excessive dust content of the clean gas will be seen at the chimney outlet.
It has been found that relatively large quantities of dust are reagitated in such cases and that such dust cannot be recollected in one or more downstream fields although the discharge of dust from the gas passages involved is highly restricted by the inhibiting gas stream which shuts off the passages.
Particularly when peak dust loadings resulting from reagitation flow in a downstream field through a gas passage which is defined by collecting electrode walls which are about to be cleaned so that their collecting capacity is reduced, or in case of a cumulation of peak dust loadings when collecting electrode walls lying one behind the other are cleaned at the same time by coincidence, intolerably high dust concentrations may occur from time to time in the clean gas.
Problems will also arise in connection with the cleaning in the last field because dust which has been reagitated in such field cannot be collected in a succeeding field.